Hydrocarbon bearing formations are frequently underlaid by a water zone. In some instances, for example, the underlying water zone, due primarily to hydrostatic pressures, exerts an upwardly directional force upon hydrocarbons within the hydrocarbon bearing formation. Generally, the pooling of hydrocarbons responding to hydrostatic forces is referred to as a bottom water drive reservoir.
In these instances, upon penetrating the hydrocarbon bearing formation by a well bore, the hydrostatic forces enhance initial hydrocarbon production. However, as hydrocarbon production continues, encroachment of water into the hydrocarbon bearing formation adjacent the well bore, a process generally referred to as "water coning", occurs. It is generally thought that water coning is dependent on capillary forces of the formation at the water/oil interface and the presence of a low pressure area around the well bore caused by hydrocarbon production therethrough. Accelerated production rates or high hydrocarbon flow rates into the well bore enhance the onset of water coning. In some instances, advanced water coning may result in substantial quantities of water accompanying the producing hydrocarbons to the surface. In some instances, economics may dictate abandoning the well where high water to oil ratios (WOR) appear in production fluids.
Because water coning generally occurs in the immediate vicinity of the perforated well bore, traditional methods for alleviating the influx of water have involved alterations of the well bore or of the properties of the formation adjacent said well bore. Plugging the perforations producing water or cement squeezing have met with limited success. Methods involving the placement of a chemical barrier at the oil/water interface have produced more favorable results.
One particular method of placing a chemical barrier at the oil/water interface is commonly referred to as "dual injection". Dual injection is generally performed by lowering a retainer, such as a packer, by means of a drill string, into the well bore until the packer is positioned adjacent to, or slightly above the oil/water interface. After securing the packer within the well bore a compatible non-damaging, non-gelling fluid, generally referred to as the balancing fluid, and a sealant are simultaneous injected into the formation. Typical sealants include aqueous mixtures of silicates, delayed metal complex polyacrylamides, in situ polymerizable acrylamides, xanthans, epoxies, etc. Generally, the balancing fluids are aqueous compositions of either formation water or brine previously produced, formation crude oil, diesel fuel or synthetic brines containing either potassium chloride, ammonium chloride or sodium chloride.
The balancing fluid is pumped down the annulus between the treating string and the casing while a sealant is pumped down the treating string. The balancing fluid generally exits the well bore and enters the hydrocarbon bearing formation above the oil/water interface while the sealant enters said formation at or below said interface. Injection pressures are balanced such that both fluids are directed radially away from the well bore while generally maintaining a substantially horizontal interface between the balancing fluid and sealant. In this way, the balancing fluid forms a temporary radial barrier extending from the well bore into the formation that prevents sealant migration into the hydrocarbon bearing formation. Once the sealant has cured, the balancing fluid is back-produced and the well is returned to production.
Occasionally, formations adjacent the oil/water interface are very permeable. In these instances, the injection volume of balancing fluid must be increased to maintain pressure equilibration of the two fluids. In some instances, fluid loss to the formation is so great that equilibration between the injection pressures of the two fluids can not be maintained by increasing the injection rate of the balancing fluid. In these instances, the radial barrier of balancing fluid may not extend a sufficient distance from the well bore. This may result in inefficient sealant placement or the sealant migration into the hydrocarbon bearing formation. In other instances, increasing the balancing fluid injection volume may result in increased operational cost and a longer back-production interval upon returning the well on-line.
Thus, improvement in this area is needed to ensure adequate sealant coverage while minimizing the loss of balancing fluid to porous formations. Under high permeable formation conditions, it has been found that by incorporating a viscosifying agent or a permeability reducing agent into the balancing fluid, a reduction in the volume thereof while maintaining pressure equilibration between the sealant and balancing fluid is observed. The results are the creation of a more effective temporary radial pressure barrier extending from the well bore and efficient sealant placement without significant adverse migration.